This paper presents the results of an effort to label calcium carbonates formed by marine organisms with stable isotopes to obtain information about the biomineralization processes. The growing skeleton of the scleractinian coral Porites porites was labeled three times with enhanced abundances of Sr-86. The distribution of Sr-86 in the skeleton was imaged with the NanoSIMS ion microprobe with a spatial resolution of similar to 200 nm and combined with images of the skeletal ultra-structure. Importantly, the distribution of the Sr-86 label in the P. porites skeleton was found to be strongly heterogeneous. This constrains the physical dimensions of the hypothetical Extracellular Calcifying Fluid (ECF) reservoir at the surface of the growing skeleton, which is implicit in most geochemical models for coral biomineralization. These new experimental capabilities allow for a much more detailed view of the growth dynamics for a wide range of marine organisms that biomineralize carbonate structures. Citation: Houlbreque, F., A. Meibom, J.-P. Cuif, J. Stolarski, Y. Marrocchi, C. Ferrier-Pages, I. Domart-Coulon, and R. B. Dunbar ( 2009), Strontium-86 labeling experiments show spatially heterogeneous skeletal formation in the scleractinian coral Porites porites, Geophys. Res. Lett., 36, L04604, doi: 10.1029/2008GL036782.